fix: send dont_have when retrieval immediately fails

[rvagg]

Maybe an alternative to just doing #169. I wonder if not playing nicely with clients will be impacting what we're receiving?

Sending immediate dont_haves rather than relying on the AwaitGet call to return for cases where the indexer says there might help since they are the most common responses.

I'm doing something like this in #167 already.

[codecov-commenter]

Codecov Report

Base: 4.87% // Head: 4.86% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (6b0ae1f) compared to base (0eeef5d).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #170      +/-   ##
=========================================
- Coverage    4.87%   4.86%   -0.02%     
=========================================
  Files          15      15              
  Lines        1723    1727       +4     
=========================================
  Hits           84      84              
- Misses       1634    1638       +4     
  Partials        5       5              

Impacted Files	Coverage Δ
bitswap/provider.go	0.00% <0.00%> (ø)

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